Steel Sheet for Formed Member Having Enhanced Ductility, Formed Member, and Method for Manufacturing the Formed Member

ABSTRACT

Provided is a steel sheet for a formed member having enhanced ductility, a formed member, and a method for manufacturing the formed member, and more particularly, to a steel sheet for making a formed member having high strength and ductility such as an automotive structural member and a reinforcing member, a formed member, and a method for manufacturing the formed member. The steel sheet for a formed member has enhanced ductility, and includes, by weight %, C: 0.1% to 1.0%, Si+Al: 0.4% to 3.0%, Mn: 0.1% to 5.0%, P: 0.0001% to 0.1%, S: 0.0001% to 0.03%, N: 0.03% or less (but not 0%), and the balance of Fe and inevitable impurities. The formed member having high strength and ductility that can be used for forming an automotive structural member, a reinforcing member, etc. In addition, the formed member can be used for a heat-treatable impact resistant member.

TECHNICAL FIELD

The present invention relates to a steel sheet for a formed member having enhanced ductility, a formed member, and a method for manufacturing the formed member, and more particularly, to a steel sheet for making a formed member having high strength and ductility such as an automotive structural member and a reinforcing member, a formed member, and a method for manufacturing the formed member.

BACKGROUND ART

As safety requirements and weight-reduction demands made on automobiles increase, research has been conducted into developing high strength and impact-resistant steel sheets as automotive body materials. However, high-strength steel sheets for automobiles may be difficult to form into desired shapes, and thus there is a limit on increasing the strength of steel sheets for automobiles.

To address these problems, Korean Patent Application Publication No. 2007-0110914 discloses a steel sheet having enhanced formability and 800-MPa grade tensile strength through the transformation of retained austenite into martensite. However, large amounts of substances such as carbon (C) and manganese (Mn) have to be added to increase the strength of the steel sheet up to 1000 MPa grade, which increases costs and abrasion of cold forming dies and makes it difficult to correct the shape of the steel sheet.

As advanced techniques proposed to solve the problems, Korean Patent Application Publication No. 2007-0057689 and U.S. Pat. No. 6,296,805 disclose techniques of performing a heat treatment and a pressing process on a steel sheet in a single phase of austenite having low strength but high formability, and then rapidly cooling the steel sheet using dies so as to implement martensite as a main microstructure in the steel sheet and to provide ultra-high strength. However, steel sheets obtained by the techniques have low degrees of elongation at less than 10%, and thus cannot have sufficient resistance against impacts generating in automobiles.

DISCLOSURE Technical Problem

Aspects of the present invention provide a steel sheet for a formed member having ultra high strength and ductility by a heat treatment, a formed member made of the steel sheet, and a method for manufacturing the formed member.

Technical Solution

According to an aspect of the present invention, there is provided a steel sheet for a formed member having enhanced ductility, the steel sheet including, by weight %, C: 0.1% to 1.0%, Si+Al: 0.4% to 3.0%, Mn: 0.1% to 5.0%, P: 0.0001% to 0.1%, S: 0.0001% to 0.03%, N: 0.03% or less (but not 0%), and the balance of Fe and inevitable impurities.

The steel sheet may further include at least one selected from the group consisting of (a), (b), (c), (d), and (e), wherein: (a) at least one selected from the group consisting of Cr: 0.01% to 2.0%, Mo: 0.01% to 2.0%, and W: 0.01% to 2.0%; (b) at least one selected from the group consisting of Ti: 0.001% to 0.4%, Nb: 0.001% to 0.4%, Zr: 0.001% to 0.4%, and V: 0.001% to 0.4%; (c) at least one of Cu: 0.005% to 2.0% and Ni: 0.005% to 2.0%; (d) B: 0.0001% to 0.01%; and (e) Sb: 0.001% to 0.1%.

The steel sheet may be one of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet. The plated steel sheet may be a hot-rolled or cold-rolled steel sheet on which a metal or resin coating layer is formed. The metal coating layer may be formed by one of aluminum plating, zinc plating, alloyed zinc plating, and zinc electroplating. The resin coating layer may be formed of at least one of solgel and aluminum powder.

After a heat treatment, the steel sheet may have a microstructure including by area fraction: 5% to 40% of retained austenite, 60% to 95% of martensite, and 10% or less (including 0%) of ferrite.

According to another aspect of the present invention, there is provided a formed member having enhanced ductility, the formed member including, by weight%, C: 0.1% to 1.0%, Si+Al: 0.4% to 3.0%, Mn: 0.1% to 5.0%, P: 0.0001% to 0.1%, S: 0.0001% to 0.03%, N: 0.03% or less (but not 0%), and the balance of Fe and inevitable impurities, wherein the formed member has a microstructure including by area fraction: 5% to 40% of retained austenite, 60% to 95% of martensite, and 10% or less (including 0%) of ferrite.

The formed member may further include at least one selected from the group consisting of (a), (b), (c), (d), and (e), wherein: (a) at least one selected from the group consisting of Cr: 0.01% to 2.0%, Mo: 0.01% to 2.0%, and W: 0.01% to 2.0%; (b) at least one selected from the group consisting of Ti: 0.001% to 0.4%, Nb: 0.001% to 0.4%, Zr: 0.001% to 0.4%, and V: 0.001% to 0.4%; (c) at least one of Cu: 0.005% to 2.0% and Ni: 0.005% to 2.0%; (d) B: 0.0001% to 0.01%; and (e) Sb: 0.001% to 0.1%.

The formed member may have a tensile strength of 1000 Mpa or greater and an elongation of 10% or greater.

According to another aspect of the present invention, there is provide a method for manufacturing a formed member, the method including: performing a pressing process on a steel sheet including, by weight%, C: 0.1% to 1.0%, Si+Al: 0.4% to 3.0%, Mn: 0.1% to 5.0%, P: 0.0001% to 0.1%, S: 0.0001% to 0.03%, N: 0.03% or less (but not 0%), and the balance of Fe and inevitable impurities; quenching the steel sheet to an Ms-Mf temperature range; and heating the quenched steel sheet to a temperature range equal to or lower than Ac1 and maintaining the steel sheet within the temperature range.

The steel sheet may further include at least one selected from the group consisting of (a), (b), (c), (d), and (e), wherein: (a) at least one selected from the group consisting of Cr: 0.01% to 2.0%, Mo: 0.01% to 2.0%, and W: 0.01% to 2.0%; (b) at least one selected from the group consisting of Ti: 0.001% to 0.4%, Nb: 0.001% to 0.4%, Zr: 0.001% to 0.4%, and V: 0.001% to 0.4%; (c) at least one of Cu: 0.005% to 2.0% and Ni: 0.005% to 2.0%; (d) B: 0.0001% to 0.01%; and (e) Sb: 0.001% to 0.1%.

Before performing the pressing process, the method may further include heating the steel sheet to an austenite temperature range equal to or higher than Ac3. After performing the pressing process, the method may further include heating the steel sheet to an austenite temperature range equal to or higher than Ac3. The quenching may be performed at a rate of 10° C./s to 500° C./s. The maintaining of the steel sheet within the temperature range may continue for 1 second to 10000 seconds.

Advantageous Effects

The present invention provides a formed member having high strength and ductility that can be used for forming an automotive structural member, a reinforcing member, etc. In addition, the formed member can be used for a heat-treatable impact resistant member.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating process conditions of the present invention.

FIG. 2( a) is a microstructure image of Comparative Specimen 6 of an example of the present invention.

FIG. 2( b) is a microstructure image of Inventive Specimen 7 of the example of the present invention.

BEST MODE

According to the present invention, a formed member having improved elongation is manufactured using a steel sheet by properly selecting the composition of the steel sheet, quenching the steel sheet to an Ms-Mf temperature range, and heating the steel sheet to be within a temperature range equal to or lower than Ac1 and maintaining the steel sheet within the temperature range. In the related art, a high strength member is manufactured using a steel sheet by performing a hot forming process on the steel sheet and then quenching the hot-formed steel sheet to a temperature lower than Mf so that martensite can become the main microstructure of the steel sheet. In the present invention, however, the steel sheet is cooled to the Ms-Mf temperature range in which transformation to martensite continues to retain a sufficient amount of austenite in the microstructure of the steel sheet, and the retained austenite is properly maintained at a temperature equal to or lower than Ac1 to concentrate carbon of the martensite on the retained austenite, which makes the retained austenite stable when the steel sheet is cooled again so that some of the retained austenite can stably reach room temperature without transformation while the remainder of the retained austenite transforms to martensite.

The present invention will now be described in more detail.

In the following description, a formed member means a member formed by deforming the steel sheet into a desired shape and includes any members used in fields requiring the physical properties of the steel sheet of the present invention.

C: 0.1% to 1.0% (hereinafter, % refer to weight %)

C is an essential element for increasing the strength of the steel sheet, and in the present invention it is required to add C to the steel sheet to appropriately retain austenite in the microstructure of the steel sheet. If the concentration of C in the steel sheet is lower than 0.1%, sufficient strength may not be obtained although the steel sheet is heat-treated in a single phase austenite region, and it is difficult to retain 5% or more of austenite when performing a heat treatment on the steel sheet after a hot forming process or a cold forming process. If the concentration of C in the steel sheet is greater than 1.0%, the toughness and welding characteristics of the steel sheet may deteriorate. In addition, when pickling and rolling a hot-rolled steel sheet, it is difficult to weld the steel sheet. Furthermore, when an annealing process or a plating process is performed, it is difficult to deform the steel sheet by cold rolling because the steel sheet may have too much high strength. Therefore, the concentration of C may be maintained in the range of 0.1% to 1.0%.

Si+Al: 0.4% to 3.0%

Si and Al are important elements in the present invention. When maintaining the steel sheet at a temperature equal to or lower than Acl after quenching the steel sheet to a temperature between Ms and Mf, Si and Al prevent the precipitation of carbon from martensite so that a large amount of carbon can be concentrated on non-transformed and retained austenite and thus the austenite can be stably retained in a final product member. If the total concentration of Si and Al is less than 0.4%, such effects may not be obtained, and if the total concentration of Si and Al exceeds 3%, it is difficult to remove scale from a hot-rolled steel sheet, and when manufacturing a formed member, a heat treatment has to be performed at a higher temperature, which increases manufacturing costs. Therefore, the concentration of Si and Al may be set to be within the range of 0.4% to 3%.

Mn: 0.1% to 5.0%

Mn is a solid-solution strengthening element which increases the strength of the steel sheet, delays transformation of austenite to ferrite, and lowers the Ac3 temperature. If the concentration of Mn is less than 0.1%, a high-temperature heat treatment is necessary to treat the steel sheet in a single phase austenite region. However, a high-temperature heat treatment increases oxidation of the steel sheet, and when the steel sheet is plated, a high-temperature heat treatment decreases the heat resistance of the steel sheet even though the steel sheet is plated. In addition, although a heat treatment is performed on the steel sheet in a two phase region where ferrite and austenite coexist, the steel sheet may not have a desired degree of strength. If the concentration of Mn exceeds 5.0%, welding and hot-rolling characteristics of the steel sheet may deteriorate. Therefore, the concentration of Mn may be in the range of 0.1% to 5.0%.

P: 0.0001% to 0.1%

Like Si, P prevents generation of carbides when martensite is heat-treated. However, if the concentration of P is excessive, welding characteristics of the steel sheet deteriorate, and thus the upper limit of the concentration of P is set to be 0.1%. In addition, since it may be relatively expensive to adjust the concentration of P to be lower than 0.0001%, the lower limit of the concentration of P may be set to be 0.0001%.

S: 0.0001% to 0.03%

S exists in the steel sheet as an impurity and worsens the ductility and welding characteristics of the steel sheet. If the concentration of S is 0.03% or lower, such undesirable effects are negligible, and thus the upper limit of the concentration of S may be set to be 0.03%. In addition, since it may be relatively expensive to to adjust the concentration of S to be lower than 0.0001%, the lower limit of the concentration of S may be set to be 0.0001%.

N: 0.03% or lower (not including 0%)

N is an element inevitably included in steel. N forms nitrides to increase resistance against delayed fracture caused by hydrogen. In addition, solute N increases the yield strength of the steel sheet when a bake hardening treatment is performed on the steel sheet after painting. If the concentration of N exceeds 0.03%, slabs may become sensitive to cracks and bubbles may be easily generated in slabs during a continuous casting process. Therefore, the concentration of N may be set to be 0.03% or lower. For example, the concentration of N may be set to be 0.02% or lower. In another example, the concentration of N may be set to be 0.01% or lower.

The steel sheet including the above-listed concentrations of elements may further include: 0.01% to 2.0% of at least one element selected from the group consisting of Cr, Mo, and W which improve hardenability; 0.001% to 0.4% of at least one element selected from the group consisting of Ti, Nb, Zr, and V which improve precipitation hardening; 0.005% to 2.0% of at least one element of Cu and Ni which improve strength; 0.0001% to 0.01% of B which improves grain boundary hardening and hardenability; or 0.001% to 0.1% of Sb which improves plating characteristics.

Cr, Mo, W: 0.01% to 2.0%

Cr, Mo, and W improve the hardenability of steel and are effective for increasing the strength of the steel. In addition, even in the case that steel is not sufficiently cooled because of unstable contact with dies during a high-temperature forming process, the steel can have proper strength owing to hardenability improved by Cr, Mo, and W. If the concentration of Cr, Mo, or W is lower than 0.01%, sufficient hardenability may not be obtained, and even in the case that the concentration of Cr, Mo, or W is greater than 2.0%, more effects are not obtained therefrom, but costs may be increased. Therefore, the concentration of Cr, Mo, or W may be set to be within the range of 0.01% to 2.0%.

Ti, Nb, Zr, V: 0.001% to 0.4%

Ti, Nb, Zr, and V make steel stronger, smaller in grain size, and have good heat treatment characteristics. If the concentration of Ti, Nb, Zr, or V is 0.001% or less, such effects may not be obtained, and if the concentration of Ti, Nb, Zr, or V is greater than 0.4%, costs may increase unnecessarily. Therefore, the concentration thereof may range from 0.001% to 0.4%.

Cu, Ni: 0.005% to 2.0%

Cu precipitates in micro sizes to increase the strength of steel. If the concentration of Cu is less than 0.005%, sufficient strength may not be obtained, and if the concentration of Cu is greater than 2.0%, workability may deteriorate. Ni improves the strength and heat-treatment characteristics of steel. If the concentration of Ni is less than 0.005%, such effects may not be obtained, and if the concentration of Ni is greater than 2.0%, costs increase. Therefore, the concentration of each of Cu and Ni may be set to be within the range of 0.005% to 2.0%.

B: 0.0001% to 0.01%

B improves hardenability, and even in the case that a small amount of B is added to steel, the strength of the steel can be markedly increased after a heat treatment. If the concentration of B is less than 0.0001%, this effect may not be obtained, and even in the case that the concentration of B is greater than 0.01%, the effect may not be further improved and hot forming characteristics may deteriorate. Therefore, the concentration of B may be set to be within the range of 0.0001% to 0.01%.

Sb: 0.001% to 0.1%

Sb concentrates on the surface of the steel sheet, and thus during annealing, Si and Al added in the steel sheet can be prevented from concentrating on the surface of the steel sheet as Si and Al oxides which deteriorate plating characteristics of the steel sheet. If the concentration of Sb is less than 0.001%, such effects may not be obtained, and if the concentration of Sb is greater than 0.1%, the hot forming characteristics of the steel sheet deteriorate. Therefore, the concentration of Sb may be set to be within the range of 0.001% to 0.1%.

A formed member of the present invention may include: 5% to 40% by area fraction of retained austenite; 60% to 95% by area fraction of martensite; and 10% or less by area fraction of ferrite (including 0%). If the area fraction of retained austenite is less than 5%, the formed member may not have high elongation which is an object of the present invention. On the contrary, adjusting the area fraction of retained austenite to be greater than 40% requires the addition of large amounts of C, Si, and Al, which makes it difficult to manufacture the formed member.

As mentioned above, the formed member may include martensite in addition to the retained austenite. Martensite may be generated when the formed member is quenched to a temperature range of Ms to Mf. In addition, martensite may be generated when the formed member is cooled after being heat- treated at a temperature lower than Ac1. At this time, carbide may generated in the martensite. The formed member may include 60% to 95% by area fraction of the martensite. If the area fraction of the martensite is less than 60%, the formed member may not have a desired degree of strength, and if the area fraction of the martensite is greater than 95%, the area fraction of the retained austenite becomes insufficient. The martensite may include non-tempered martensite and tempered martensite.

If the formed member is heat-treated at a temperature between Ac1 and Ac3 or is slowly cooled after a heat treatment, ferrite may be generated. The area fraction of the ferrite in the formed member may be set to not exceed 10%. If the area fraction of the ferrite exceeds 10%, the formed member may not have a desired degree of strength.

An object of the present invention is to provide a steel sheet having a tensile strength of 1000 Mpa or greater and elongation of 10% or greater by adjusting the composition and microstructure of the steel sheet as described above and manufacturing the steel sheet as described below. In addition, a formed member having high strength and ductility can be provided using the steel sheet. Upper limits of the tensile strength and elongation of the steel sheet are not mentioned because it is preferable that the steel sheet have as much tensile strength and elongation as possible.

A method for manufacturing a steel sheet such as a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet will now be explained according to the present invention.

A steel slab having the above-described composition is heated at 1000° C. to 1300° C. and is hot-rolled. If the heating temperature is lower than 1000° C., the continuous-cast structure of the slab may not become uniformed, and if the heating temperature is higher than 1300° C., manufacturing costs may increase. Then, the slab undergoes a finish hot rolling process at a temperature between Ar3 and 1000° C. to form a rhot-olled steel sheet. If the process temperature of the finish hot rolling process is lower than Ar3, the hot rolling process may become a two-phase-region rolling process to mix grains of the hot-rolled steel sheet and make it difficult to perform the process. If the process temperature of the finish hot rolling process is higher than 1000° C., grains of the hot-rolled steel sheet may coarsen. Thereafter, the hot-rolled steel sheet is coiled at a temperature higher than Ms but equal to or lower than 750° C. If the hot-rolled steel sheet is coiled at a temperature of Ms or lower, transformation to martensite occurs to excessively increase the strength of the hot-rolled steel sheet, and if the hot-rolled steel sheet is coiled at a temperature higher than 750° C, the thickness of an oxide layer of the hot-rolled steel sheet increases. The hot-rolled steel sheet may be heat-treated according to the use thereof or may be pickled and used to form a formed member. After such a heat treatment or pickling process, the hot-rolled steel sheet may be plated to improve corrosion resistance. Then, the plated hot-rolled steel sheet may be used for form a formed member. The plating process is not limited to a particular type or method. Any proper plating types and methods known in the related art may be used for the plating process.

The hot-rolled steel sheet is pickled and cold-rolled. The reduction ratio of the cold rolling is not limited, for example, may be a commonly used value. The cold-rolled steel sheet is continuously annealed or batch-annealed. Process conditions of the annealing process are not limited. For example, the strength of the cold-rolled steel sheet may be reduced through the annealing process for the next cold forming process or heat treatment. As described above, the cold-rolled steel sheet may be plated. The plating process is not limited to a particular type or method because the plating process has substantially no influence on the properties of the cold-rolled steel sheet that are required in the present invention.

For example, the plating process for the cold-rolled steel sheet or hot-rolled steel sheet may be an aluminum plating process, a zinc plating process, an alloyed zinc plating process, or a zinc electroplating process. Instead of the plating process, a resin coating process may be performed by using at least one of solgel and aluminum powder.

As described above, the steel sheet may include by area fraction: 5% to 40% of retained austenite; 60% to 95% of martensite; and 10% or less (including 0%) of ferrite.

A method for manufacturing a formed member will now be described.

The steel sheet manufactured as described above undergoes a pressing process in which the steel sheet is hot-pressed or cold-pressed and is heat-treated. The hot pressing is performed after the steel sheet is heated to an austenite region temperature equal to or higher than Ac3. However, in the case of the cold pressing, the steel sheet is cold-pressed and is then heated to an austenite region temperature equal to or higher than Ac3.

The pressed steel sheet is quenched to a temperature range between Ms and Mf. If the pressed steel sheet is quenched to a temperature higher than Ms, transformation to martensite may not occur but transformation to bainite may occur, and in the following cooling treatment, retained austenite may transform to martensite. In this case, the area fraction of martensite may not be 5% or higher. If the steel sheet is quenched to a temperature lower than Mf, austenite may not be retained while transformation to martensite. Therefore, even in the case that the steel sheet is maintained at a temperature equal to or lower than Ac1 for diffusing carbon, austenite may not be generated or 5% or less of austenite may be generated.

The steel sheet may be quenched to a temperature range between Ms and Mf(90%). The Mf(90%) temperature is a temperature for allowing about 10% of retained austenite. That is, if the steel sheet is quenched to a temperature higher than Mf(90%), more carbon may be concentrated in austenite, and 5% or more of retained austenite may be stably guaranteed even in the case that austenite transforms to martensite during reheating and cooling.

The quenching may be performed at a rate of 10° C./s to 500° C./s. If the rate of quenching is lower than 10° C./s, ferrite or bainite may be generated and martensite may be insufficient. If the rate of quenching is higher than 500° C./s, even in the case that martensite generate sufficiently, process costs increase due to the excessively high quenching rate.

Thereafter, the quenched steel sheet is heated to a temperature equal to or lower than Ac1 and maintained at that temperature. If the heating, that is, tempering is performed in a two-phase region including ferrite and austenite at a temperature higher than Ac1, a large amount of ferrite may be generated and martensite may be rapidly softened, and thus the steel sheet may not have desired high strength. Although the lower temperature limit of the tempering is not mentioned in the above description, if the tempering is performed at a temperature lower than Ms, it may take too many time to diffuse carbon into austenite retained in the quenching process. Therefore, the tempering may be performed at a temperature equal to or higher than Ms.

In the above, when the quenched steel sheet is heated to a temperature equal to or lower than Ac1 and maintained at that temperature, the maintaining period may be 1 second to 10000 seconds. If the maintaining period is shorter than 1 second, carbon of martensite may not sufficiently concentrate on austenite, and if the maintaining period time is longer than 10000 seconds, martensite may excessively softened to lower the strength of the steel sheet.

Mode for Invention

The present invention will now be described in more detail with reference to an example. The example is exemplary example which is intended to explain the present invention, and the present invention is not limited to the example.

EXAMPLE

Steel slabs having compositions as shown in table 1 and formed by vacuum melting were heated at 1000° C to 1300° C. for 1 hour in a heating furnace, and were hot-rolled to form hot-rolled steel sheets. The hot-rolled steel sheets were cooled at a predetermined temperature in a furnace. The hot rolling was finished at a temperature range of 850° C. to 950° C., and the cooling temperature at the furnace was 680° C. Thereafter, the hot-rolled steel sheets were pickled and cold-rolled at a reduction ratio of 50%. The cold-rolled steel sheets were annealed at 780° C. Thereafter, the cold-rolled steel sheets were finally continuously annealed at an overaging temperature of 400° C.

TABLE 1 Chemical Composition (weight %) Nos. C Si Mn P S Al N Others *IS 1 0.11 1.6 3.03 0.0027 0.0053 0.034 0.0033 — IS 2 0.31 1.6 1.54 0.0023 0.0038 0.032 0.0022 — IS 3 0.31 1.6 3.03 0.0025 0.0043 0.039 0.0153 — IS 4 0.32 1.6 3.00 0.0029 0.0054 0.032 0.0037 — IS 5 0.31 1.7 2.98 0.0035 0.0045 0.565 0.0027 — IS 6 0.31 1.1 1.54 0.0160 0.0071 0.013 0.0039 — IS 7 0.31 1.7 2.97 0.0032 0.0043 0.022 0.0040 Ti: 0.049 IS 8 0.31 1.6 3.02 0.0026 0.0045 0.026 0.0027 Nb: 0.048 IS 9 0.30 1.6 2.97 0.0042 0.0038 0.041 0.0032 V: 0.095 IS 10 0.31 1.7 3.04 0.0051 0.0027 0.034 0.0028 Zr: 0.047 IS 11 0.632 1.7 3.03 0.0027 0.0051 0.032 0.0028 B: 0.0027 IS 12 0.31 1.6 2.98 0.0031 0.0052 0.030 0.0036 Cr: 0.50 IS 13 0.30 1.5 2.99 0.0045 0.0028 0.027 0.0047 Mo: 0.08 IS 14 0.31 1.7 3.10 0.0037 0.0046 0.045 0.0033 W: 0.04 IS 15 0.29 1.6 3.03 0.0028 0.0038 0.033 0.0027 Cu: 0.12 IS 16 0.32 1.6 2.99 0.0028 0.0053 0.030 0.0035 Ni: 0.51 IS 17 0.31 1.6 3.01 0.0047 0.0034 0.027 0.0029 Sb: 0.047 **CS 0.32 0.1 3.02 0.0025 0.0054 0.033 0.0041 — 1 CS 2 0.23 0.2 1.29 0.0112 0.0022 0.031 0.0042 Ti: 0.023 B: 0.0022 *IS: Inventive Steel, **CS: Comparative Steel

A heat treatment in a hot pressing process was simulated as shown in FIG. 1 and Table 2. In detail, the steel sheets manufactured as described above were heated at a rate of 30° C./s and maintained at 900° C. (SS), higher than Ac3, for 3 minutes. Then, the steel sheets were cooled to a predetermined temperature T1 at a rate of 30° C./s and maintained for 2 seconds at the predetermined temperature T1. Thereafter, the steel sheets were reheated to a predetermined temperature T2 and maintained at the predetermined temperature T2 for 10 seconds. After that, the steel sheets were cooled to room temperature at a rate of 50° C./s. The predetermined temperatures T1 and T2 were determined by dilatation tests. Ac3, Ac1, and Ms were calculated from T1 and T2, and Mf(90%) for 90% transformation was calculated by the lever rule. Specimens were made of the steel sheets according to JIS Z 2201 No. 5 SPECIMEN, and mechanical properties of the specimens were measured. The area fraction of retained austenite was calculated by Formula 1 below and the 5-peak method using areas of austenite (200), (220), and (311) peaks, and areas of ferrite (200) and (211) peaks obtained from X-ray diffraction tests. Mechanical properties and area fractions of austenite of the specimens are shown in Table 3 below.

$\begin{matrix} \underset{\underset{6}{}}{\begin{matrix} \begin{matrix} {V_{1}^{XRD} = {\left\lbrack {{1\text{/}2.19\left( {I_{\alpha}^{200}\text{/}I_{y}^{200}} \right)} + 1} \right\rbrack + \left\lbrack {{1\text{/}1.35\left( {I_{\alpha}^{220}\text{/}I_{y}^{200}} \right)} + 1} \right\rbrack +}} \\ {\left\lbrack {{1\text{/}1.5\left( {I_{\alpha}^{200}\text{/}I_{y}^{311}} \right)} + 1} \right\rbrack + \left\lbrack {{1\text{/}1.12\left( {I_{\alpha}^{211}\text{/}I_{y}^{200}} \right)} + 1} \right\rbrack +} \end{matrix} \\ {\left\lbrack {{1\text{/}0.7\left( {I_{\alpha}^{211}\text{/}I_{y}^{200}} \right)} + 1} \right\rbrack \left\lbrack {{1\text{/}0.78\left( {I_{\alpha}^{211}\text{/}I_{y}^{311}} \right)} + 1} \right\rbrack} \end{matrix}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$

TABLE 2 Nos. Heat Treatment Conditions Steel Specimen Ac3 Ac1 Ms Mf (90%) SS T1 T2 t2 types Nos. (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) (s) *IS 1 **ISP 1 852 669 377 164 900 269 421 10 IS 2 ISP 2 845 734 338 125 900 253 416 10 IS 3 ISP 3 804 680 293 79 900 201 423 10 ISP 4 900 198 423 100 ISP 5 900 176 419 500 ISP 6 900 194 521 10 ***CSP 1 900 194 821 10 CSP 2 750 170 419 10 IS 4 ISP 7 804 682 290 76 900 185 411 10 IS 5 ISP 8 856 686 315 104 900 237 402 10 IS 6 ISP 9 828 725 345 132 900 207 423 10 CSP 3 900 238 285 10 CSP 4 900 50 — 10 IS 7 ISP 10 809 683 292 78 900 200 4118 10 IS 8 ISP 11 807 680 294 80 900 208 423 10 IS 9 ISP 12 814 681 298 84 900 218 381 10 IS 10 ISP 13 808 681 290 6 900 226 408 10 IS 11 ISP 14 805 682 288 74 900 192 420 10 IS 12 ISP 15 800 687 288 75 900 212 419 10 IS 13 ISP 16 805 680 299 87 900 195 406 10 IS 14 ISP 17 807 679 288 74 900 191 402 10 IS 15 ISP 18 807 678 300 86 900 206 437 10 IS 16 ISP 19 789 659 279 70 900 181 387 10 IS 17 ISP 20 804 681 292 78 900 198 392 10 ****CS 1 CSP 5 764 660 309 96 900 201 422 10 CS 2 CSP 6 791 686 380 169 900 215 416 10 *IS: Inventive Steel, **ISP: Inventive Specimen, ***CSP: Comparative Specimen, ****CS: Comparative Steel

TABLE 3 Microstructure Nos. (Area fraction %) Steel Specimen YP TS El *Re- **M + Types Nos. (MPa) (MPa) (%) tained γ TM ***F IS 1 ISP 1 1003 1194 13.7 9.5 88.0 2.5 IS 2 ISP 2 1019 1291 14.8 13.3 83.1 3.6 IS 3 ISP 3 971 1594 14.8 14.2 84.6 1.2 ISP 4 1063 1479 16.8 16.6 82.7 0.7 ISP 5 1213 1445 17.7 17.0 82.5 0.5 ISP 6 791 1396 14.2 12.7 86.5 0.8 CSP 1 1221 1970 1.6 3.7 95.6 0.7 CSP 2 601 1460 3.9 3.5 80.8 15.7 IS 4 ISP 7 1015 1563 17.0 15.2 84.3 0.5 IS 5 ISP 8 973 1518 18.3 16.3 79.5 4.2 IS 6 ISP 9 1342 1491 13.8 11.4 87.7 0.9 CSP 3 1263 1867 6.1 0.6 98.7 0.7 CSP 4 1327 1973 5.3 0.1 99.5 0.4 IS 7 ISP 10 1034 1601 17.6 13.3 86.2 0.5 IS 8 ISP 11 974 1591 15.2 14.3 84.8 0.9 IS 9 ISP 12 1032 1507 16.3 14.2 85.4 0.4 IS 10 ISP 13 1063 1542 16.7 14.8 84.5 0.7 IS 11 ISP 14 973 1597 17.0 14.2 83.4 2.4 IS 12 ISP 15 884 1647 15.5 15.2 83.5 1.3 IS 13 ISP 16 1106 1612 16.8 14.7 84.5 0.8 IS 14 ISP 17 1057 1586 16.9 14.8 84.9 0.3 IS 15 ISP 18 1152 1573 15.4 13.7 85.9 0.4 IS 16 ISP 19 1139 1602 17.7 16.8 82.5 0.7 IS 17 ISP 20 1053 1582 16.2 14.5 84.7 0.8 CS 1 CSP 5 1090 1406 7.4 0.2 99.4 0.4 CS 2 CSP 6 1158 1260 6.4 0.0 99.4 0.6 *Retained γ: retained austenite, **M: martensite, TM: tempered martensite, ***F: ferrite

Referring to Tables 1 to 3, Inventive Specimens 1 to 20, formed of steels having compositions and manufactured by the method according to the present invention, have 5% or more by area fraction of retained austenite and good elongation. However, Comparative Specimens 5 and 6 have less than 5% by area fraction of retained austenite and poor elongation because Comparative Specimens 5 and 6 do not have compositions according to the present invention.

Although Comparative Specimens 1 and 2 have compositions according to the present invention, T2 and SS are not within the ranges proposed by the present invention. Thus, Comparative Specimens 1 and 2 do not have 5% or more of retained austenite and thus have poor elongation.

Although Comparative Specimens 3 and 4 have compositions according to the present invention, T2 and T1 are not within the ranges proposed by the present invention. Thus, Comparative Specimens 3 and 4 have poor elongation. In the case of Comparative Specimen 3, T2 is lower than Ms, and thus a sufficient period of time and temperature cannot be ensured to stabilize retained austenite. As a result, in the final cooling process, most austenite transforms to martensite, and thus austenite cannot be sufficiently retained. In the case of Comparative Specimen 4, T1 is lower than Mf which is advantageous in terms of high strength but disadvantageous in terms of retaining a desired amount of austenite because most austenite transforms to martensite.

FIGS. 2( a) and 2(b) are microstructure images of the formed members. FIG. 2( a) is a microstructure image of Comparative Specimen 6, and FIG. 2( b) is a microstructure image of Inventive Sample 7. Referring to FIGS. 2( a) and 2(b), Comparative Specimen 6 to which sufficient Si is not added does not retain sufficient austenite, and thus, a large amount of cementite precipitates on grains or grain boundaries. However, in the case of Inventive Sample 7, most retained austenite is stabilized and present at grain boundaries because sufficient carbon diffuses during a heat treatment at

T2. 

1. A steel sheet for a formed member having enhanced ductility, the steel sheet comprising, by weight%, C: 0.1% to 1.0%, Si+Al: 0.4% to 3.0%, Mn: 0.1% to 5.0%, P: 0.0001% to 0.1%, S: 0.0001% to 0.03%, N: 0.03% or less but not 0%, and the balance of Fe and inevitable impurities.
 2. The steel sheet of claim 1, further comprising at least one selected from the group consisting of (a), (b), (c), (d), and (e), wherein: (a) at least one selected from the group consisting of Cr: 0.01% to 2.0%, Mo: 0.01% to 2.0%, and W: 0.01% to 2.0%; (b) at least one selected from the group consisting of Ti: 0.001% to 0.4%, Nb: 0.001% to 0.4%, Zr: 0.001% to 0.4%, and V: 0.001% to 0.4%; (c) at least one of Cu: 0.005% to 2.0% and Ni: 0.005% to 2.0%; (d) B: 0.0001% to 0.01%; and (e) Sb: 0.001% to 0.1%.
 3. The steel sheet of claim 1, wherein the steel sheet is one of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet.
 4. The steel sheet of claim 3, wherein the plated steel sheet is a hot-rolled or cold-rolled steel sheet on which a metal or resin coating layer is formed.
 5. The steel sheet of claim 4, wherein the metal coating layer is formed by one of aluminum plating, zinc plating, alloyed zinc plating, and zinc electroplating.
 6. The steel sheet of claim 4, wherein the resin coating layer is formed of at least one of solgel and aluminum powder.
 7. The steel sheet of claim 1, wherein after a heat treatment, the steel sheet has a microstructure comprising by area fraction: 5% to 40% of retained austenite, 60% to 95% of martensite, and 10% or less (including 0%) of ferrite.
 8. A formed member having enhanced ductility, the formed member comprising, by weight%, C: 0.1% to 1.0%, Si+Al: 0.4% to 3.0%, Mn: 0.1% to 5.0%, P: 0.0001% to 0.1%, S: 0.0001% to 0.03%, N: 0.03% or-less but not 0%, and the balance of Fe and inevitable impurities, wherein the formed member has a microstructure comprising by area fraction: 5% to 40% of retained austenite, 60% to 95% of martensite, and 10% or less (including 0%) of ferrite.
 9. The formed member of claim 8, further comprising at least one selected from the group consisting of (a), (b), (c), (d), and (e), wherein: (a) at least one selected from the group consisting of Cr: 0.01% to 2.0%, Mo: 0.01% to 2.0%, and W: 0.01% to 2.0%; (b) at least one selected from the group consisting of Ti: 0.001% to 0.4%, Nb: 0.001% to 0.4%, Zr: 0.001% to 0.4%, and V: 0.001% to 0.4%; (c) at least one of Cu: 0.005% to 2.0% and Ni: 0.005% to 2.0%; (d) B: 0.0001% to 0.01%; and (e) Sb: 0.001% to 0.1%.
 10. The formed member of claim 8, wherein the formed member has a tensile strength of 1000 Mpa or greater and an elongation of 10% or greater.
 11. A method for manufacturing a formed member, the method comprising: performing a pressing process on a steel sheet comprising, by weight%, C: 0.1% to 1.0%, Si+Al: 0.4% to 3.0%, Mn: 0.1% to 5.0%, P: 0.0001% to 0.1%, S: 0.0001% to 0.03%, N: 0.03% or less but not 0%, and the balance of Fe and inevitable impurities; quenching the steel sheet to an Ms-Mf temperature range; and heating the quenched steel sheet to a temperature range equal to or lower than Ac1 and maintaining the steel sheet within the temperature range.
 12. The method of 11, wherein the formed member further comprises at least one selected from the group consisting of (a), (b), (c), (d), and (e), wherein: (a) at least one selected from the group consisting of Cr: 0.01% to 2.0%, Mo: 0.01% to 2.0%, and W: 0.01% to 2.0%; (b) at least one selected from the group consisting of Ti: 0.001% to 0.4%, Nb: 0.001% to 0.4%, Zr: 0.001% to 0.4%, and V: 0.001% to 0.4%; (c) at least one of Cu: 0.005% to 2.0% and Ni: 0.005% to 2.0%; (d) B: 0.0001% to 0.01%; and (e) Sb: 0.001% to 0.1%.
 13. The method of claim 11, wherein before performing the pressing process, the method further comprises heating the steel sheet to an austenite temperature range equal to or higher than Ac3.
 14. The method of claim 11, wherein after performing the pressing process, the method further comprises heating the steel sheet to an austenite temperature range equal to or higher than Ac3.
 15. The method of claim 11, wherein the quenching is performed at a rate of 10 ° C./s to 500° C./s.
 16. The method of claim 11, wherein the maintaining of the steel sheet within the temperature range continues for 1 second to 10000 seconds.
 17. The steel sheet of claim 2, wherein after a heat treatment, the steel sheet has a microstructure comprising by area fraction: 5% to 40% of retained austenite, 60% to 95% of martensite, and 10% or less (including 0%) of ferrite.
 18. The steel sheet of claim 3, wherein after a heat treatment, the steel sheet has a microstructure comprising by area fraction: 5% to 40% of retained austenite, 60% to 95% of martensite, and 10% or less (including 0%) of ferrite.
 19. The steel sheet of claim 4, wherein after a heat treatment, the steel sheet has a microstructure comprising by area fraction: 5% to 40% of retained austenite, 60% to 95% of martensite, and 10% or less (including 0%) of ferrite.
 20. The steel sheet of claim 5, wherein after a heat treatment, the steel sheet has a microstructure comprising by area fraction: 5% to 40% of retained austenite, 60% to 95% of martensite, and 10% or less (including 0%) of ferrite. 